The present invention relates to a hydraulically-operated frictional engagement device formed between a first member and a second member which are adapted to be rotatable relatively to each other, more particularly to a direct-coupling clutch incorporated into a fluid torque converter.
A direct-coupling clutch is formed between the front cover of a fluid torque converter and the turbine runner or the output shaft of the fluid torque converter in order that the pump impeller of the fluid torque converter and the turbine runner or the output shaft of the fluid torque converter can be directly coupled. This direct-coupling clutch comprises an annular first friction surface formed on the inner surface of the front cover and a flat and annular friction surface formed on a clutch disk axially slidably mounted on the turbine runner or on the output shaft of the fluid torque converter and is adapted to engage the first and the second friction surfaces through a friction member made of a material of a large coefficient of friction by applying a fluid pressure to the back surface of the clutch disk.
It is known that the clutch disk is deformed slightly by the fluid pressure applied to the back surface of the clutch disk, since the clutch disk has a small thickness as compared with its large diameter. The deformation of the clutch disk results inevitably in the reduction in the effective frictional area.
Generally, it is desirable that the friction member has a large coefficient of dynamical friction to cope with considerable slip at the biginning of frictional engagement, a high heat resistance to avoid burning caused by the frictional heat generated during a transient period of engagement and a large heat capacity or a heat radiating property. On the other hand, after the frictional engagement has been completed, a large coefficient of statical friction and a high compressive strength (a strength to resist a great pressure working on the surface) are essential to provide a large capacity of torque transmission. Thus different characteristics are required of the friction member during the course of frictional engagement and after the completion of frictional engagement, therefore, a friction member made of a single material is incapable of meeting those different characteristics.